The present invention relates to a photogrammetric device for use in aircraft or spacecraft. The device generates orientation data, the course or flight path of the aircraft or spacecraft, and produces a digital terrain representation by the use of photo sensitive semiconductors arranged in lines or rows.
In the science of earth observation from the air or from space, it is customary to survey the surface of the earth being passed over by an aircraft or space vehicle by the use of linear arrays of charge-coupled elements ("CCD's") which operate as sensors. These arrays include normally ca 1700 or more elements arranged in rows. An entire row is arranged at right angles to the aircraft trajectory, and the individual elements are successively and electronically read out to provide sensed image lines. See "Interavia," December 1978, at 1160. This method provides the decided advantage of allowing a direct optoelectronic conversion of the signals generated in the sensor into digital data, and permits storage of this data on magnetic tape or the like. Also, further processing of the data in computers is then possible. With the aid of high speed computers, evaluation in real time is also possible without intermediate storage.
With this image scanning device, the coordinates of discrete image points, as well as the light intensity, can be measured in a digital manner. On the whole, the descripted technology mentioned above offers the possibility of direct transmission of the found data to the ground stations by means of telemetry and fully automated computer processing. Therefore, this technology is now used in the fields of topographic mapping, photogrammetry, navigation, distant exploration, military reconnaissance, and the like.
However, a disadvantage lies in the fact that an exact geometric evaluation of the sensed image lines is not yet possible. This is a defect inherent with the technology of line sensing generally. While a conventional photograph, which has a central perspective, yields an exact geometrical allocation across its entire area, this central perspective is not present with line sensing, i.e., the perspective is reduced to the central perspective geometry of each individual scanning line.
Each line of an image, within one image strip, is attributed with any inherent external orientation, since the sensor carrier is in motion in relation to its position as well as its inclination, and such motions are either approximately known, or not known at all. This fact constitutes a principal hurdle prohibiting a geometric evaluation. Until now, in order to solve this problem, it has been suggested to use a continuous measuring technique for the external orientation by way of external means, e.g., gyroscopic platforms, inertial methods and the like. Also, it is known to employ distortion correction methods using ground reference points.
Finally, attempts are made to predict position during flight on the basis of statistical forecasts. All of these methods have the decided disadvantage of being cumbersome and relatively expensive, because of the necessity of additional instrumentation, manual intermediate processes, or the determination of a large number of reference points.